In recent years, substantial interest has grown in motor vehicles having suspension systems which can offer improved comfort and road handling for the vehicle. The improvements for these suspension systems can be achieved by utilization of an “intelligent” suspension system. An “intelligent” suspension is capable of controlling the suspension forces generated by the hydraulic dampers or shock absorbers provided at each corner of the motor vehicle in response to one or more operating characteristics of the vehicle.
In general, vehicle suspension systems are provided to filter or isolate the vehicle body from road surface irregularities as well as to control body and wheel motion. In addition, it may be desirable that the suspension system maintain an average vehicle attitude to promote improved vehicle stability during maneuvering. The conventional non-intelligent suspension system includes a spring and a damping device in parallel which is located between the sprung mass (vehicle body) and the unsprung mass (the wheel and suspension systems).
Hydraulic actuators, such as shock absorbers and/or struts, are used in conjunction with the conventional non-intelligent or passive suspension systems to absorb unwanted vibrations which occur during driving. To absorb these unwanted vibrations, the conventional hydraulic actuators often include a piston which is located within a pressure tube and which is connected to the body of the automobile through a piston rod. The pressure tube is connected to the vehicle's suspension system. Because the piston is able to limit the flow of damping fluid within the working chamber of the pressure tube when the actuator is telescopically displaced, the actuator is able to produce a damping force which counteracts the vibration which would otherwise be directly transmitted from the suspension system to the vehicle body. The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the actuator.
In order to maintain a vehicle's attitude for multiple loading conditions, it is often desirable to have a leveling system associated with the vehicle. This vehicle leveling system can be associated with the hydraulic damper, it can be associated with the vehicle's springs, or it may be separate from both the hydraulic dampers and the springs. These leveling systems are used to change the relationship between the vehicle's suspension system and the vehicle body. The leveling systems are used to compensate for weight changes associated with the vehicle. The weight changes can be the result of changes in static loading or changes in dynamic loading. Static loading is simply the load which is supported by the suspension system which is due to the weight associated with the passengers of the vehicle, the weight of the cargo in the vehicle, and the like. In contrast, dynamic loading involves the loading which normally varies according to different types of road conditions.
When changing the height of the vehicle in response to the vehicle's weight, it is also desirable to adjust the damping characteristics of the vehicle's hydraulic dampers. A relatively low weighted vehicle typically requires a softer or lower damping characteristic than a relatively highly weighted vehicle. The continued development of suspension systems have been directed towards methods of adjusting the damping characteristics of the hydraulic dampers in response to the leveling system which reacts to the vehicle's weight.